Are you prohibited from making a U-turn from the sign to the closest intersection inclusive?

Why?

The B-23 no U-turn sign applies from its location up to and including the next junction. You therefore cannot make a U-turn either before that junction or at it. The green traffic light in the distance does not cancel this restriction.

§ 15 ust. 2 i § 22 ust. 5 rozporządzenia w sprawie znaków i sygnałów drogowych